D - electron availability

Table 24.3 lists representative examples of the compounds of these elements in their various oxidation states. The wide range of the oxidation states is particularly noteworthy. It arises from the fact that, in moving across the transition series, the number of d electrons has increased and, in this mid-region, the d orbitals have not yet sunk energetically into the inert electron core. The number of d electrons available for bonding is consequently maximized, and not... 

Consider now NajW03 or LiTi204. One might expect to find W(V)-W(VI) and Ti(III)-Ti(lV) MMCT. However, in these compounds all metal ions are equivalent and the d electrons available are spread out in a conduction band. The bronzes NajW03 are metallic. They become superconducting at 6K, whereas LiTi204 becomes superconducting at even 13 K [59]. Here we meet the central problem of mixed-valence compounds [60] which we will postpone till Sect. 5. 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

When the charge on the electrode is made negative, the bond is weakened due to donation of charge from the metal into adsorbate x orbitals and the band frequency shifts to lower wavenumber. When the charge on the metal is made positive a shift to higher frequency occurs. At a mercury electrode, however, there are no p- or d-electrons available to participate in a back-bonding interaction. 

According to conventional wisdom transition metals with empty d shells should have spherically symmetric electron densities since there are no d electrons available to cause a distortion. They should therefore have regular coordination environments but, surprisingly, the largest electronic distortions are shown by six-coordinate d° or d cations. Transition-metal cations with empty d shells... 

Calculated lattice energies (using estimated ionization potentials where necessary) increase in the order BaS, Ce8, Th8, that is, as the number of d electrons available for metallic bonding increases from 0 to 1 to 2. The bond lengths M—8 show a decrease in the same order in both MS and M2S3 ... 

The potentials of CO2 reduction is well correlated with the heat of fusion of the electrode metals (Fig. 9). The heat of fusion is related to the extent of d electron contribution to metallic bond °, and may be taken as a measure of d electron a ilability d electron availability will affect the back donation and thus determine the extent of the stabilization of adsorbed CO2 . Stabilized CO2 will have the extra negative charge on O atoms like CO2 coordinating... 

Ahrland, Chatt, and Davies explained the class (b) metals as having d electrons available for TT bonding. Elements farther left in the table have more class (b) character in low... 

The structures of ordered TiO and of NbO are closely related to the rocksalt structure but with 1/6 and 1/4 of the respective positions of both M and O atoms being unoccupied. The reduced lattice energy due to the vacancies has to be compensated for by M-M bonding. The proportion of vacancies therefore increases with the number of d electrons available for M-M bonding. The rock-salt structure is stable for d systems (e. g. TiN, NbC, etc), the structure of the TiO d system lies between the rocksalt structure and the structure of the NbO d system. 

---